The present invention relates to electron discharge devices and particularly to a high speed cage for an electron multiplier.
An electron multiplier is a device utilizing secondary electron emission to amplify or multiply the electron current from a primary electron source, such as a photocathode or a thermionic cathode. The usual electron multiplier comprises a series or chain of secondary emitting elements, called dynodes, interposed between a primary electron source and an output collector or anode. The electrodes are constructed and arranged to form an electron optical system for directing primary electrons from the primary source onto the first dynode, releasing therefrom several secondary electrons for each primary electron. These secondaries are directed by the electron optical system onto the next dynode where each produces more secondaries. This process is repeated at each succeeding dynode or "stage" of the multiplier, thus producing a greatly multiplied electronic current from the final dynode to the collector. The number of dynodes or stages may be from one to twenty or more depending on the amount of amplification needed. Each succeeding dynode in the chain is maintained at a potential substantially higher, e.g., 100 volts, than the preceding dynode, to accelerate the secondaries from element to element and the dynodes are preferably shaped to direct and focus the electrons emitted thereby to the next dynode.
Electron multipliers are particularly useful for amplifying electron currents produced by weak signals, such as light or nuclear radiation. When used for detecting and/or counting rapidly recurrent signals such as nuclear particles, it is necessary that the multiplier have sufficient speed and a resolving time low enough to distinguish between successive signals or particles.
The speed of a multiplier can be increased by reducing the overall transit time of primary and secondary electrons between the primary source and the collector. The resolving time of a multiplier is limited by the transit time spread of electrons through the multiplier chain, that is, the difference between the transit times of the fastest and slowest electrons. This transit time spread is primarily due to differences in the trajectories taken by various electrons through the multiplier and differences in the initial velocities of secondary electrons.
In photomultiplier tubes, the speed or transit time of the tube is a function of both the photocathode transit time difference and the transit time of the electron multiplier. The photocathode transit time difference, defined as the time difference between peak current outputs for simultaneous small-spot illumination of different parts of the photocathode, is longer for edge illumination than for center illumination because of the longer edge trajectories and the weaker electric field near the edge of the photocathode. In a planar photocathode, the center-to-edge transit time difference may be as much as 10 nanoseconds; whereas for sphericalsection photocathodes, such as that shown in FIG. 1, the transit time response is more uniform because the electron paths are nearly equal in length.
The photocathode transit time difference is ultimately limited by the initial velocity distribution and angular distribution of the photoelectrons. These distributions cause time broadening of the electron packet during its flight from the photocathode to the first dynode. The broadening effect can be minimized by increasing the strength of the electric field at the surface of the photocathode.
Since the energy spread of secondary electrons is even larger than that of photoelectrons, the initial velocity distribution of the photoelectrons is one of the major limitations of the time response of the electron multiplier. Among the expedients used to improve the time response of the electron multiplier are the use of high electric field strengths at the dynode surfaces and compensated dynode design geometries.
A portion of a compensated design multiplier is shown in the prior art structure of FIG. 2. In a compensated design multiplier longer electron paths and weaker electric fields alternate with shorter electron paths and stronger fields from dynode to dynode to produce nearly equal total transit time for the secondary electrons.
U.S. Pat. No. 2,200,722 to Pierce et al., issued on May 14, 1940 and U.S. Pat. No. 2,245,624 to Teal, issued June 17, 1941 are representative of structures utilizing centrally disposed auxiliary electrodes between the rows of dynodes for producing strong electric fields to accelerate and converge the secondary electrons from one dynode in the multiplier to the succeeding dynode. The auxiliary electrodes are disclosed to be linear rods or wires which are operated at potentials higher than either of the dynodes bracketing the auxiliary electrode.
U.S. Pat. No. 2,868,994 to Anderson, issued on Jan. 13, 1959 and U.S. Pat. No. 2,903,595 to Morton, issued on Sept. 8, 1959 disclose electron multipliers having high voltage accelerating and focusing electrodes which operate at potentials several hundred to several thousand volts more positive than the potentials on the adjacent dynodes. In an embodiment shown in FIG. 1 of the Morton patent, a high voltage apertured accelerating electrode 15 comprises grids or otherwise apertured portions 17 connected together at their ends to form a single zig-zag member that extends along the medial plane between the two rows of dynodes.
The high voltage focusing and accelerating electrode structures described above reduce the secondary electron transit time through the electron multiplier; however, such electrodes frequently generate extraneous noise because of the great potential difference between the electrodes and the adjacent dynodes.